The present disclosure generally relates to communications systems and more particularly to wireless transceivers.
Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
In wireless transceivers, a radio frequency (RF) front end includes a matching network that provides impedance matching between an antenna and either the transmitter or receiver. FIG. 1 depicts an example of a conventional transceiver 100. Transceiver 100 includes a matching network 102, a power amplifier (PA) 104, a low noise amplifier (LNA) 106, and an antenna 108. A transmitter includes PA 104 and a receiver includes LNA 106. A front end of transceiver 100 includes matching network 102.
Matching network 102 may be a balun or any matching network, such as an inductor capacitor (LC), T-matching or π-matching network, etc. Matching network 102 is inserted between an antenna 108 and PA 104/LNA 106.
Matching network 102 provides an impedance transformation such that a load of Zload is transformed to achieve the desired impedance for the impedance transformation with Zs, the impedance of antenna 108. Zload is the impedance of either PA 104 or LNA 106 looking into matching network 102.
Different matching networks 102 may be used. FIG. 2 depicts an example of a conventional balun 202 being used as a matching network. Balun 202 transforms Zload to the desired impedance. For example, if the desired optimum loading for Zload for PA 1-104 is 100 ohm, while the antenna impedance, Zs, is 50 ohm, then the impedance ratio of inductors 204a and 204b is L1:L2=1:2.
If the 1:2 ratio is desired for PA 1-104, a drawback of a shared front end for PA 104 and LNA 1-106 is that there is less flexibility in designing for the optimum LNA loading. For example, if PA 1-104 transmits with a larger output power, which requires a smaller loading resistance, Zload, then the impedance ratio between L1 and L2 is small. This causes a small matching network gain for LNA 1-106 in a receive path. A small matching network gain may result in a poor noise figure in signals received. With a shared front end, the receive path performance will have to be compromised because LNA 1-106 shares balun 202 with PA 1-104. Thus, the design of balun 202 cannot be varied and the receive path uses an impedance ratio of 1:2 even though the ratio may not be optimal for the receiver.
Balun 202 may also be designed with an impedance ratio for optimum receiver performance by having Zload be higher. However, the transmitted output power may not be optimized because of the high impedance. Thus, having a matching network that is shared does not optimize both transmitter and receiver performance.
A balun may be designed with three inductors to allow slight variance in transmit and receive characteristics. FIG. 3 shows an example of a conventional three inductor balun 300.
Balun 300 includes three inductors 308a-308c. Balun 300 may tap inductors 308b and 308c at different points. In this way, the transmitter and receiver may be designed with different characteristics. For example, PA 1-104 taps balun 300 at points 302a and 302b. Also, LNA 1-106 taps balun 300 at points 304a and 304b. There are two inductors—L1 and L2. L1 and L2 couple from antenna 1-108 to LNA 1-106 and achieve the designated impedance transformation. L3 and L1 couple from PA 1-104 to antenna 1-108 and achieve the designated impedance transformation. This architecture may achieve the independent impedance transformation in the receiver and antenna, and transmitter and antenna by varying the tapping point of L3 from L2. However, there are multiple drawbacks. For example, there is poor isolation between the receiver and transmitter. Also, it may be less flexible to tap out at ports 302a and 302b due to the limitation of the inductor geometry. Also, any trace tapping out from ports 302a and 302b will form extra parasitic inductance on the lines from PA 1-104 and LNA 1-106 to inductor L2.